Method and apparatus for transmitting signal in wireless communication system

ABSTRACT

A method and apparatus for transmitting a signal in a wireless communication system are provided. The method includes: generating R spatial streams each of which is generated on the basis of an information stream and a reference signal; generating N transmit streams on the basis of the R spatial streams and a precoding matrix (where R&lt;N); mapping the N transmit streams to at least one resource block; and generating N signals from the N transmit streams mapped to the at least one resource block, and transmitting the N signals to a user equipment through respective antennas.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for transmitting a signal in awireless communication system.

BACKGROUND ART

Wireless communication systems are widely spread all over the world toprovide various types of communication services such as voice or data.The wireless communication system is designed for the purpose ofproviding reliable communication to a plurality of users irrespective oftheir locations and mobility. However, a wireless channel has anabnormal characteristic such as a fading phenomenon caused by a pathloss, noise, and multipath, an inter-symbol interference (ISI), aDoppler effect caused by mobility of a user equipment, etc. Therefore,various techniques have been developed to overcome the abnormalcharacteristic of the wireless channel and to increase reliability ofwireless communication.

A multiple input multiple output (MIMO) scheme is used as a techniquefor supporting a reliable high-speed data service. The MIMO scheme usesmultiple transmit antennas and multiple receive antennas to improve datatransmission/reception efficiency. Examples of the MIMO scheme includespatial multiplexing, transmit diversity, beamforming, etc.

A MIMO channel matrix is formed by multiple receive antennas andmultiple transmit antennas. A rank can be obtained from the MIMO channelmatrix. The rank is the number of spatial layers. The rank may also bedefined as the number of spatial streams that can be simultaneouslytransmitted by a transmitter. The rank is also referred to as a spatialmultiplexing rate. If the number of transmit antennas is Nt and thenumber of receive antennas is Nr, a rank R satisfies R≦min{Nt, Nr}.

A wireless communication system requires a signal known to both atransmitter and a receiver to perform channel measurement, informationdemodulation, or the like. The signal known to both the transmitter andthe receiver is referred to as a reference signal (RS). The RS may alsobe referred to as a pilot.

The receiver may estimate a channel between the transmitter and thereceiver by using the RS, and may demodulate information by using theestimated channel. When a user equipment receives an RS transmitted by abase station, the user equipment may measure a channel by using the RS,and may feed back channel state information to the base station.

A signal transmitted from the transmitter experiences a channelcorresponding to each transmit antenna or each spatial layer, and thusthe RS may be transmitted for each transmit antenna or each spatiallayer. If the RS is transmitted for each spatial layer, the RS may betransmitted after precoding. In this case, the receiver needs to knowinformation on a frequency region in which the same precoding matrix isused.

Accordingly, there is a need to provide a method and apparatus foreffectively transmitting a signal in a wireless communication system.

SUMMARY OF INVENTION Technical Problem

The present invention provides a method and apparatus for transmitting asignal in a wireless communication system.

Solution to Problem

According to one aspect of the present invention, a signal transmissionmethod in a wireless communication system is provided. The methodincludes: generating R spatial streams each of which is generated on thebasis of an information stream and a reference signal; generating Ntransmit streams on the basis of the R spatial streams and a precodingmatrix (where R<N); mapping the N transmit streams to at least oneresource block; and generating N signals from the N transmit streamsmapped to the at least one resource block, and transmitting the Nsignals to a user equipment through respective antennas.

According to another aspect of the present invention, a signaltransmission apparatus in a wireless communication system is provided.The apparatus includes: N antennas; and a processor coupled to the Nantennas and configured for transmitting precoding bandwidth informationindicating a bandwidth in which the same precoding matrix is used;generating R spatial streams each of which is generated on the basis ofan information stream and a reference signal; generating N transmitstreams on the basis of the R spatial streams and a precoding matrix;mapping the N transmit streams to at least one resource block; andgenerating N signals from the N transmit streams mapped to the at leastone resource block, and transmitting the N signals to a user equipmentthrough respective antennas.

Advantageous Effects of Invention

According to the present invention, a method and apparatus foreffectively transmitting a signal in a wireless communication system areprovided. Therefore, overall system performance can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows an exemplary structure of a radio frame.

FIG. 3 shows an example of a resource grid for one downlink slot.

FIG. 4 shows an exemplary structure of a downlink subframe.

FIG. 5 shows exemplary mapping of a common reference signal (RS) for oneantenna when using a normal cyclic prefix (CP).

FIG. 6 shows exemplary mapping of common RSs for two antennas when usinga normal CP.

FIG. 7 shows exemplary mapping of common RSs for four antennas whenusing a normal CP.

FIG. 8 shows exemplary mapping of a common RS for one antenna when usingan extended CP.

FIG. 9 shows exemplary mapping of common RSs for two antennas when usingan extended CP.

FIG. 10 shows exemplary mapping of common RSs for four antennas whenusing an extended CP.

FIG. 11 shows exemplary mapping of a dedicated RS in a long termevolution (LTE) when using a normal CP.

FIG. 12 shows exemplary mapping of a dedicated RS in an LTE when usingan extended CP.

FIG. 13 is a block diagram showing an exemplary structure of atransmitter.

FIG. 14 is a block diagram showing an exemplary structure of aninformation processor of FIG. 13.

FIG. 15 is a block diagram showing an exemplary structure of atransmitter for generating a non-precoded dedicated RS.

FIG. 16 is a block diagram showing an exemplary structure of atransmitter for generating a precoded dedicated RS.

FIG. 17 is a block diagram showing an exemplary apparatus for wirelesscommunication using a precoded dedicated RS.

FIG. 18 is a flowchart showing a signal transmission method in awireless communication system according to an embodiment of the presentinvention.

FIG. 19 shows an example of a feedback subband when using a singleprecoding matrix indicator (PMI) type.

FIG. 20 shows an example of a precoding subband when using a single PMItype.

FIG. 21 shows an example of a feedback subband when using a multiple PMItype

FIG. 22 shows an example of a precoding subband when using a multiplePMI type.

FIG. 23 shows an example of a precoding bandwidth.

FIG. 24 is a block diagram showing an apparatus for wirelesscommunication for implementing an embodiment of the present invention.

MODE FOR THE INVENTION

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc.The UTRA is a part of a universal mobile telecommunication system(UMTS). 3^(rd) generation partnership project (3GPP) long term evolution(LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPPLTE uses the OFDMA in a downlink and uses the SC-FDMA in an uplink.LTE-advance (LTE-A) is an evolution of the LTE.

For clarity, the following description will focus on the LTE(Release8)/LTE-A(Release 10). However, technical features of the presentinvention are not limited thereto.

FIG. 1 shows a wireless communication system.

Referring to FIG. 1, a wireless communication system 10 includes atleast one base station (BS) 11. Respective BSs 11 provide communicationservices to specific geographical regions (generally referred to ascells) 15 a, 15 b, and 15 c. The cell can be divided into a plurality ofregions (referred to as sectors). A user equipment (UE) 12 may be fixedor mobile, and may be referred to as another terminology, such as amobile station (MS), a user terminal (UT), a subscriber station (SS), awireless device, a personal digital assistant (PDA), a wireless modem, ahandheld device, an access terminal (AT), etc. The BS 11 is generally afixed station that communicates with the UE 12 and may be referred to asanother terminology, such as an evolved node-B (eNB), a base transceiversystem (BTS), an access point, an access network (AN), etc.

Hereinafter, a downlink (DL) implies communication from the BS to theUE, and an uplink (UL) implies communication from the UE to the BS. Inthe DL, a transmitter may be a part of the BS, and a receiver may be apart of the UE. In the UL, the transmitter may be a part of the UE, andthe receiver may be a part of the BS.

The wireless communication system can support multiple antennas. Thetransmitter may use a plurality of transmit antennas, and the receivermay use a plurality of receive antennas. The transmit antenna denotes aphysical or logical antenna used for transmission of one signal orstream. The receive antenna denotes a physical or logical antenna usedfor reception of one signal or stream. When the transmitter and thereceiver use a plurality of antennas, the wireless communication systemmay be referred to as a multiple input multiple output (MIMO) system.

A wireless communication process is preferably implemented with aplurality of independent hierarchical layers rather than one singlelayer. A structure of a plurality of hierarchical layers is referred toas a protocol stack. The protocol stack may refer to an open systeminterconnection (OSI) model which is a widely known protocol forcommunication systems.

FIG. 2 shows an exemplary structure of a radio frame.

Referring to FIG. 2, the radio frame consists of 10 subframes, and onesubframe consists of two slots. Slots included in the radio frame areindexed with slot numbers from #0 to #19. A time for transmitting onesubframe is defined as a transmission time interval (TTI). The TTI maybe regarded as a scheduling unit for information transmission. Forexample, one radio frame may have a length of 10 milliseconds (ms), onesubframe may have a length of 1 ms, and one slot may have a length of0.5 ms. The structure of the radio frame is for exemplary purposes only,and thus the number of subframes included in the radio frame or thenumber of slots included in the subframe may change variously.

FIG. 3 shows an example of a resource grid for one DL slot.

Referring to FIG. 3, the DL slot includes a plurality of orthogonalfrequency division multiplexing (OFDM) symbols in a time domain, andincludes N_DL resource blocks (RBs) in a frequency domain. The OFDMsymbol is for expressing one symbol period, and may also be referred toas another terminology, such as an OFDMA symbol, an SC-FDMA symbol,etc., according to a multiple access scheme. The number N_DL of RBsincluded in the DL slot depends on a downlink transmission bandwidthdetermined in a cell. In an LTE, the number N_DL may be any value in therange of 6 to 110. One RB includes a plurality of subcarriers in thefrequency domain.

Each element on the resource grid is referred to as a resource element.The resource element on the resource grid can be identified by an indexpair (k, l) in the slot. Herein, k(k=0, . . . , N^(DL)×12−1) denotes asubcarrier index in the frequency domain, and l(l=0, . . . , 6) denotesan OFDM symbol index in the time domain.

Although it is described herein that one RB includes 7 (12 resourceelements consisting of 7 OFDM symbols in the time domain and 12subcarriers in the frequency domain for example, the number of OFDMsymbols and the number of subcarriers included in the RB are not limitedthereto. The number of OFDM symbols may variously change depending on acyclic prefix (CP) length and a subcarrier spacing. For example, in caseof a normal CP, the number of OFDM symbols is 7, and in case of anextended CP, the number of OFDM symbols is 6.

The resource grid for one DL slot of FIG. 3 can also apply to a resourcegrid for a UL slot.

FIG. 4 shows an exemplary structure of a DL subframe.

Referring to FIG. 4, the DL subframe includes two consecutive slots.First 3 OFDM symbols of a 1^(st) slot included in the DL subframecorrespond to a control region, and the remaining OFDM symbolscorrespond to a data region. Herein, the control region includes 3 OFDMsymbols for exemplary purposes only.

A physical downlink shared channel (PDSCH) may be allocated to the dataregion. DL data is transmitted over the PDSCH.

A control channel may be allocated to the control region. Examples ofthe control channel include a physical control format indicator channel(PCFICH), a physical HARQ indicator channel (PHICH), a physical downlinkcontrol channel (PDCCH), etc.

The PCFICH carries information indicating the number of OFDM symbolsused for transmission of PDCCHs in a subframe to a UE. The number ofOFDM symbols used for PDCCH transmission may change in every subframe.The PHICH carries HARQ acknowledgement (ACK)/negative acknowledgement(NACK) for UL data.

The PDCCH carriers DL control information. Examples of the DL controlinformation include DL scheduling information, UL schedulinginformation, or a UL power control command, etc. The DL schedulinginformation is also referred to as a DL grant. The UL schedulinginformation is also referred to as a UL grant.

The DL grant may include a resource allocation field indicating atime-frequency resource for transmitting DL data, a modulation andcoding scheme (MCS) field indicating an MCS level of the DL data, etc.

If a transmission scheme is multiple user-MIMO (MU-MIMO), the DL grantmay further include a power offset field. The power offset fieldindicates power offset information for obtaining downlink transmissionenergy for each resource element.

The transmission scheme is a scheme in which a BS transmits DL data tothe UE.

Examples of the transmission scheme include a single antenna scheme, aMIMO scheme, etc. The MIMO scheme includes a transmit diversity scheme,a closed-loop spatial multiplexing scheme, an open-loop spatialmultiplexing scheme, a MU-MIMO system, etc. The transmission scheme maybe semi-statically determined using higher layer signaling such as radioresource control (RRC) signaling.

A wireless communication system requires a signal known to both atransmitter and a receiver to perform channel measurement, informationdemodulation, or the like. The signal known to both the transmitter andthe receiver is referred to as a reference signal (RS). The RS may alsobe referred to as a pilot. The RS does not carry information derivedfrom a higher layer, and may be generated in a physical layer.

When the RS is transmitted, the RS may be multiplied by a pre-determinedRS sequence. The RS sequence may be a binary sequence or a complexsequence. For example, the RS sequence may use a pseudo-random (PN)sequence, an m-sequence, etc. However, this is for exemplary purposesonly, and thus there is no particular restriction on the RS sequence.When the BS transmits the RS by multiplying the RS by the RS sequence,the UE can reduce interference acting on the RS by a signal of aneighbor cell. Accordingly, channel estimation performance can beimproved.

The RS can be classified into a common RS and a dedicated RS.

The common RS is an RS transmitted to all UEs in a cell. All UEs in thecell may receive the common RS. To avoid inter-cell interference, thecommon RS may be determined in a cell-specific manner. In this case, thecommon RS is referred to as a cell-specific RS. The common RS may beused in channel estimation and information demodulation. An example ofan RS used only for channel measurement includes a channel stateinformation-RS (CSI-RS).

The dedicated RS is an RS received by a specific UE group or a specificUE in a cell. The dedicated RS cannot be used by other UEs. Thededicated RS is also referred to as a UE-specific RS. The dedicated RSmay be transmitted using a resource block allocated for DL datatransmission of the specific UE. The dedicated RS may be used ininformation demodulation.

FIG. 5 shows exemplary mapping of a common RS for one antenna when usinga normal CP. FIG. 6 shows exemplary mapping of common RSs for twoantennas when using a normal CP. FIG. 7 shows exemplary mapping ofcommon RSs for four antennas when using a normal CP. FIG. 8 showsexemplary mapping of a common RS for one antenna when using an extendedCP. FIG. 9 shows exemplary mapping of common RSs for two antennas whenusing an extended CP. FIG. 10 shows exemplary mapping of common RSs forfour antennas when using an extended CP.

Referring to FIG. 5 to FIG. 10, Rp denotes a resource element used forRS transmission through an antenna #p (where p=0, 1, 2, 3). Hereinafter,the resource element used for RS transmission is referred to as areference resource element. The resource element Rp is defined as areference resource element for the antenna #p. The resource element Rpis used only for transmission through the antenna #p, and is not usedfor any other transmissions. In other words, a resource element used forRS transmission through a certain antenna in a subframe is not used forany other transmissions through other antennas in the same subframe, andmay be set to ‘0’. This is to avoid interference between antennas.

For convenience of explanation, a minimum unit of an RS pattern in atime-frequency resource is hereinafter referred to as a basic unit. TheRS pattern determines a location of a reference resource element in thetime-frequency resource. If the basic unit is extended to a time domainand/or a frequency domain, the RS pattern is iterated. Herein, the basicunit is one subframe in the time domain and one resource block in thefrequency domain.

A common RS may be transmitted in every DL subframe. One common RS istransmitted for each antenna. The common RS corresponds to a set ofreference resource elements in a subframe. A BS may transmit the commonRS by multiplying the common RS by a pre-defined common RS sequence.

An RS pattern of the common RS is referred to as a common RS pattern.Common RS patterns for the respective antennas are orthogonal to eachother in the time-frequency domain. The common RS pattern is common toall UEs in a cell. The common RS sequence is also common to all UEs inthe cell. However, to minimize inter-cell interference, each of thecommon RS pattern and the common RS sequence may be determined in acell-specific manner.

The common RS sequence may be generated on an OFDM symbol basis in onesubframe. The common RS sequence may differ according to a cellidentifier (ID), a slot number in one radio frame, an OFDM symbol indexin a slot, a CP length, etc.

In an OFDM symbol including a reference resource element in a basicunit, the number of reference resource elements for one antenna is 2.That is, in an OFDM symbol including the resource element Rp in thebasic unit, the number of resource elements Rp is 2. A subframe includesN_DL resource blocks in the frequency domain. Therefore, in an OFDMsymbol including the resource element Rp in the subframe, the number ofresource elements Rp is 2(N_DL. Further, in the OFDM symbol includingthe resource element Rp in the subframe, a length of a common RSsequence for the antenna #p is 2(N_DL.

The following equation shows an example of a complex sequence r(m)generated for a common RS sequence in one OFDM symbol.

$\begin{matrix}{{{r(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{11mu},{2\; {N\_ max}},{{DL} - 1}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

Herein, N_max,DL denotes the number of resource blocks corresponding toa maximum downlink transmission bandwidth supported in the wirelesscommunication system. In an LTE, N_max,DL is 110. If N_DL is less thanN_max,DL, a certain part of a length of 2(N_DL may be used as a commonRS sequence by being selected from a complex sequence generated in alength of 2(N_max,DL. c(i) denotes a PN sequence. The PN sequence can bedefined by a gold sequence having a length of 31. The following equationshows an example of c(i).

c(n)=(x(n+Nc)+y(n+Nc))mod 2

x(n+31)=(x(n+3)+x(n))mod 2

y(n+31)=(y(n+3)+y(n+2)+x(n+1)+x(n)) mod 2   [Math.2]

Herein, Nc is 1600, x(i) is a first m-sequence, and y(i) is a secondm-sequence. For example, the first m-sequence may be initialized tox(0)=1, x(i)=0(i=1, 2, . . . , 30) in the beginning of each OFDM symbol.The second m-sequence may be initialized in the beginning of each OFDMsymbol according to a cell ID, a slot number in a radio frame, an OFDMsymbol index in a slot, a CP length, etc.

The following equation shows an example of initialization of the secondm-sequence.

$\begin{matrix}{{\sum\limits_{i = 0}^{30}{{y(i)} \cdot 2^{i}}} = {{2^{10}\left( {{7\left( {{n\_ s} + 1} \right)} + l + 1} \right)\left( {{2\; {N\_ cell}{\_ ID}} + 1} \right)} + {2\; {N\_ cell}{\_ ID}} + {N\_ CP}}} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

Herein, n_s denotes a slot number in a radio frame, l denotes an OFDMsymbol index in a slot, and N_cell_ID denotes a cell ID. In case of anormal CP, N_CP is 1. In case of an extended CP, N_CP is 0.

When the common RS sequence is generated according to the aforementionedequations, the common RS sequence is irrelevant to antennas. Therefore,if the common RS is transmitted for each of a plurality of antennas inthe same OFDM symbol, each of the plurality of antennas uses the samecommon RS sequence.

The common RS sequence generated for each OFDM symbol including thereference resource element is mapped to the reference resource elementaccording to a common RS pattern. The common RS sequence may besequentially mapped to the reference resource element in the frequencydomain in an ascending order of a subcarrier index in the N_DL resourceblocks. That is, the common RS may be transmitted throughout a fullfrequency band. In this case, the common RS sequence is generated foreach antenna, and the common RS sequence is mapped to the referenceresource element for each antenna.

FIG. 11 shows exemplary mapping of a dedicated RS in an LTE when using anormal CP. FIG. 12 shows exemplary mapping of a dedicated RS in an LTEwhen using an extended CP.

Referring to FIG. 11 and FIG. 12, R5 denotes a resource element used fordedicated RS transmission through an antenna #5. In the LTE, thededicated RS is supported for single antenna transmission. Only whensingle antenna transmission through the antenna #5 is determined by ahigher layer as DL data transmission over a PDSCH, the dedicated RS canexist and be useful for PDSCH demodulation. The dedicated RS may betransmitted only over a resource block to which the PDSCH is mapped. Thededicated RS corresponds to a set of reference resource elements in theresource block to which the PDSCH is mapped. A BS may transmit thededicated RS by multiplying the dedicated RS by a pre-defined dedicatedRS sequence. Herein, a basic unit is one subframe in a time domain andone resource block in a frequency domain.

The dedicated RS may be transmitted simultaneously with the common RS.Therefore, an RS overhead becomes significantly greater in comparisonwith an RS overhead for a case where only the common RS signal istransmitted. A UE may use the common RS and the dedicated RS together.In a control region for transmitting control information in a subframe,the UE uses the common RS. In a data region existing in the subframeother than the control region, the UE may use the dedicated RS. Forexample, the control region consists of OFDM symbols of which an OFDMsymbol index l is 0 to 2 in a first slot of the subframe (see FIG. 4).

A dedicated RS pattern is an RS pattern of a dedicated RS and may becommon to all UEs in a cell. However, to minimize inter-cellinterference, the dedicated RS pattern may be determined in acell-specific manner. The dedicated RS sequence may be determined in aUE-specific manner. Therefore, only a specific UE in the cell canreceive the dedicated RS.

The dedicated RS sequence may be generated on a subframe basis. Thededicated RS sequence may differ according to a cell ID, a subframelocation in one radio frame, a UE ID, etc.

The number of reference resource elements for the dedicated RS in abasic unit is 12. That is, the number of resource elements R5 in thebasic unit is 12. If N_PDSCH denotes the number of resource blocks towhich the PDSCH is mapped, the total number of resource elements R5 forthe dedicated RS is 12(N_PDSCH. Therefore, a length of the dedicated RSsequence is 12(N_PDSCH. The length of the dedicated RS sequence maydiffer according to the number of resource blocks allocated to the UEfor PDSCH transmission.

The following equation shows an example of a dedicated RS sequence r(m).

$\begin{matrix}{{{r(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{11mu},{{12 \times {N\_ PDSH}} - 1}} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack\end{matrix}$

Herein, c(i) denotes a PN sequence. c(i) may be determined by Equation 2above. In this case, the second m-sequence may be initialized in thebeginning of each subframe according to a cell ID, a subframe locationin one radio frame, a UE ID, etc.

The following equation shows an example of initialization of the secondm-sequence.

$\begin{matrix}{{\sum\limits_{i = 0}^{30}{{y(i)} \cdot 2^{i}}} = {{\left( {\left\lfloor {{n\_ s}/2} \right\rfloor + 1} \right) \cdot \left( {{2\; {N\_ cell}{\_ ID}} + 1} \right) \cdot 2^{16}} + {UE\_ ID}}} & \left\lbrack {{Math}.\mspace{14mu} 5} \right\rbrack\end{matrix}$

Herein, n_s denotes a slot number in a radio frame, N_cell_ID denotes acell ID, and UE_ID denotes a UE ID.

The dedicated RS sequence is mapped to the reference resource elementaccording to the RS pattern in a resource block to which the PDSCH ismapped. In this case, the dedicated RS sequence is sequentially mappedin the resource block in an ascending order of a subcarrier, and is thenmapped to a reference resource element in an ascending order of an OFDMsymbol index.

Although the LTE supports the dedicated RS for single spatial stream orsingle antenna transmission as described above, an LTE-A also has tosupport the dedicated RS for multiple spatial stream transmission ormultiple antenna transmission. Therefore, there is a need to provide amethod and apparatus for transmitting a dedicated RS for multiplespatial stream transmission or multiple antenna transmission.

Hereinafter, a method and apparatus for transmitting information and adedicated RS through multiple antennas will be described. The followingdescription can also apply not only to the LTE-A system but alto to aconventional OFDM-MIMO system.

FIG. 13 is a block diagram showing an exemplary structure of atransmitter. The transmitter may be a part of a UE or a BS.

Referring to FIG. 13, a transmitter 100 includes an informationprocessor 110, Nt resource element mappers 120-1, . . . , 120-Nt, NtOFDM signal generators 130-1, . . . , 130-Nt, Nt radio frequency (RF)units 140-1, . . . , 140-Nt, and Nt transmit antennas 190-1, . . . ,190-Nt (where Nt is a natural number).

The information processor 110 is coupled to each of the Nt resourceelement mappers 120-1, . . . , 120-Nt. The Nt resource element mappers120-1, . . . , 120-Nt are respectively coupled to the Nt OFDM signalgenerators 130-1, . . . , 130-Nt. The Nt OFDM signal generators 130-1, .. . , 130-Nt are respectively coupled to the Nt RF units 140-1, . . . ,140-Nt. The Nt RF units 140-1, . . . , 140-Nt are respectively coupledto the Nt transmit antennas 190-1, . . . , 190-Nt. That is, a resourceelement mapper #n 120-n is coupled to an OFDM signal generator #n 130-n,the OFDM symbol generator #3 130-n is coupled to an RF unit #n 140-n,and the RF unit #n 140-n is coupled to a transmit antenna #n 190-n(where n=1, . . . , Nt). In case of multiple antenna transmission, oneresource grid is defined for each transmit antenna.

Information is input to the information processor 110. The informationmay be control information or data. The information may have a format ofa bit or a bit-stream. The transmitter 100 can be implemented in aphysical layer. In this case, the information may be derived from ahigher layer such as a medium access control (MAC) layer.

The information processor 110 is configured to generate Nt transmitstreams #1, #2, . . . , #Nt from the information. Each of the Nttransmit streams includes a plurality of transmit symbols. The transmitsymbol may be a complex-valued symbol obtained by processing theinformation.

The Nt resource element mappers 120-1, . . . , 120-Nt are configured toreceive the respective Nt transmit streams. That is, the resourceelement mapper #n 120-n is configured to receive a transmit stream #n.The resource element mapper #n 120-n is configured to map the transmitstream #n to resource elements in a resource block allocated forinformation transmission. Each transmit symbol of the transmit stream #nmay be mapped to one resource element. ‘0’ may be inserted to a resourceelement to which the transmit stream #n is not mapped.

One or more resource blocks may be allocated for informationtransmission. If a plurality of resource blocks are allocated, theplurality of resource blocks may be allocated either continuously ordiscontinuously.

Each of the Nt OFDM signal generators 130-1, . . . , 130-Nt isconfigured to generate a time-continuous OFDM signal for each OFDMsymbol. The time-continuous OFDM signal is also referred to as an OFDMbaseband signal. Each of the Nt OFDM signal generators 130-1, . . . ,130-Nt may generate an OFDM signal by performing inverse fast Fouriertransform (IFFT), CP insertion, or the like on each OFDM symbol.

Each of the Nt RF units 140-1, . . . , 140-Nt converts its OFDM basebandsignal into a radio signal. The OFDM baseband signal may be convertedinto the radio signal by performing up-conversion at a carrierfrequency. The carrier frequency is also referred to as a centerfrequency. The transmitter 100 may use either a single carrier ormultiple carriers.

Radio signals are respectively transmitted through the Nt transmitantennas 190-1, . . . , 190-Nt.

FIG. 14 is a block diagram showing an exemplary structure of theinformation processor of FIG. 13.

Referring to FIG. 14, an information processor 200 includes Q channelencoders 210-1, . . . , 210-Q, Q scramblers 220-1, . . . , 220-Q, Qmodulation mappers 230-1, . . . , 230-Q, a layer mapper 240, and aprecoder 250.

The Q channel encoders 210-1, . . . , 210-Q are respectively coupled tothe Q scramblers 220-1, . . . , 220-Q. The Q scramblers 220-1, . . . ,220-Q are respectively coupled to the plurality of modulation mappers230-1, . . . , 230-Q. The plurality of modulation mappers 230-1, . . . ,230-Q are coupled to the layer mapper 240. The layer mapper 240 iscoupled to the precoder 250.

The precoder 250 is coupled to Nt resource element mappers (see FIG. 5).That is, a channel encoder #q 210-q is coupled to a scrambler #q 220-q,and the scrambler #q 220-q is coupled to a modulation mapper #q 230-q(where q=1, . . . , Q).

Each of the Q channel encoders 210-1, . . . , 210-Q is configured toreceive information bit, and to generate an encoded bit by performingchannel coding on the information bit. The information bit correspondsto information to be transmitted by a transmitter. A size of theinformation bit may be various according to the information. A size ofthe encoded bit may also be various according to the size of theinformation bit and a channel coding scheme. There is no restriction onthe channel coding scheme. Examples of the channel coding scheme mayinclude turbo coding, convolution coding, block coding, etc. An encodedbit obtained by performing channel coding on the information bit isreferred to as a codeword. Herein, Q denotes the number of codewords.The channel encoder #q 210-q outputs a codeword #q (where q=1, . . . ,Q).

Each of the Q scramblers 220-1, . . . , 220-Q is configured to generatea scrambled bit for each codeword. The scrambled bit is generated byscrambling the encoded bit with a scrambling sequence. The scrambler #q220-q is configured to generate a scrambled bit for the codeword #q(where q=1, . . . , Q).

Each of the Q modulation mappers 230-1, . . . , 230-Q is configured togenerate a modulation symbol for each codeword. The modulation symbolmay be complex-valued symbol. The modulation mapper #q 230-q isconfigured to generate a modulation symbol by mapping the scrambled bitfor the codeword #q to a symbol for representing a location on a signalconstellation (where q=1, . . . , Q). There is no restriction on amodulation scheme. For example, the modulation scheme may be m-phaseshift keying (PSK) or m-quadrature amplitude modulation (QAM). Thenumber of modulation symbols output from the modulation mapper #q 230-qfor the codeword #q may be various according to a size of the scrambledbit and the modulation scheme.

The layer mapper 240 is configured to map a modulation symbol for eachcodeword to R spatial layers. The modulation symbol may be mapped to thespatial layers in various manners. R spatial streams are generated as aresult. Herein, R denotes a rank. The rank R may be equal to or greaterthan the number Q of codewords.

The precoder 250 is configured to generate Nt transmit streams byperforming precoding on the R spatial streams. The number Nt of transmitantennas is equal to or less than the rank R.

The Nt transmit streams generated by the precoder 250 are respectivelyinput to the Nt resource element mappers (see FIG. 5). The Nt transmitstreams are respectively transmitted through the Nt transmit antennas.That is, a transmit stream #n is input to a resource element mapper #n,and is transmitted through a transmit antenna #n (where n=1, 2, . . . ,Nt).

As such, a MIMO scheme in which multiple spatial streams aresimultaneously transmitted through a plurality of transmit antennas isreferred to as spatial multiplexing. The spatial multiplexing includessingle-user spatial multiplexing and multi-user spatial multiplexing.The single-user spatial multiplexing is referred to as single user(SU)-MIMO. The multi-user spatial multiplexing is referred to as multiuser (MU)-MIMO. The MU-MIMO can be supported in both uplink and downlinktransmissions.

In case of the SU-MIMO, a plurality of spatial layers are all allocatedto one UE. Through the plurality of spatial layers allocated to one UE,the multiple spatial streams are transmitted by using the sametime-frequency resource.

In case of the MU-MIMO, a plurality of spatial layers are allocated to aplurality of UEs. The multiple spatial streams allocated to theplurality of UEs are transmitted by using the same time-frequencyresource. A different spatial layer is allocated to a different UE. If Rdenotes a rank, R spatial streams can be allocated to K UEs (where2(K(R, and K is a natural number). Each of the K UEs simultaneouslyshares a time-frequency resource used for multiple spatial streamtransmission.

A dedicated RS for multiple antenna transmission may be either aprecoded RS or a non-precoded RS.

The non-precoded RS is an RS which is always transmitted by the numberof transmit antennas irrespective of the number of spatial layers. Thenon-precoded RS has an independent RS for each transmit antenna. Ingeneral, the common RS is the non-precoded RS. This is because aprecoder is generally used for a specific UE. However, if acell-specific precoder exists in a specific system, virtualization isconsidered rather than precoding.

The precoded RS is an RS which is transmitted by the number of spatiallayers. The precoded RS has an independent RS for each spatial layer.

FIG. 15 is a block diagram showing an exemplary structure of atransmitter for generating a non-precoded dedicated RS.

Referring to FIG. 15, a transmitter 300 includes a layer mapper 310, aprecoder 320, an RS generator 330, and Nt resource element mappers340-1, . . . , 340-Nt. Herein, Nt denotes the number of transmitantennas of the transmitter 300. Although not shown in FIG. 15, thestructures of FIG. 13 and FIG. 14 may be used by reference in thedescription of the structure of the transmitter 300. It is assumed thatthe number of spatial layers is R.

The layer mapper 310 is coupled to the precoder 320. Each of theprecoder 320 and the RS generator 330 is coupled to the Nt resourceelement mappers 340-1, . . . , 340-Nt.

The layer mapper 310 is configured to generate R spatial streams SS #1,SS #2, . . . , SS #R for the R spatial layers.

The precoder 320 is configured to generate Nt transmit streams TS #1, TS#2, . . . , TS #Nt by performing precoding on the R spatial streams.

The RS generator 330 generates an RS sequence in association with an RS.The RS sequence consists of a plurality of reference symbols. Anysequence may be used for the RS sequence, without any particularlyrestriction.

The RS generator 330 is configured to generate an RS sequence for eachof the Nt transmit antennas. The RS generator 330 is configured togenerate Nt RS sequences RS #1, RS #2, . . . , RS #Nt. Each of the Nt RSsequences includes a plurality of RS symbols. The RS symbol may be acomplex-valued symbol.

Each of the Nt resource element mappers 340-1, . . . , 340-Nt isconfigured to receive a transmit stream and an RS sequence and to mapthe transmit stream and the RS sequence to resource elements. A resourceelement mapper #n 340-n may receive a TS #n and an RS #n and map them toresource elements (where n=1,2, . . . , Nt).

FIG. 16 is a block diagram showing an exemplary structure of atransmitter for generating a precoded dedicated RS.

Referring to FIG. 16, a transmitter 400 includes a layer mapper 410, anRS generator 420, a precoder 430, and Nt resource element mappers 440-1,. . . , 440-Nt. Herein, Nt denotes the number of transmit antennas ofthe transmitter 400. Although not shown in FIG. 16, the structures ofFIG. 13 and FIG. 14 may be used by reference in the description of thestructure of the transmitter 400. It is assumed that the number ofspatial layers is R.

Each of the layer mapper 410 and the RS generator 420 is coupled to theprecoder 430. The precoder 430 is coupled to the Nt resource elementmappers 440-1, . . . , 440-Nt. The layer mapper 410 is configured togenerate R information streams. The R information streams can beexpressed by IS #1, IS #2, . . . , IS #R.

The RS generator 420 is configured to generate R RS sequences. The R RSsequences can be expressed by RS #1, RS #2, . . . , RS #R. Each of the RRS sequences includes a plurality of RS symbols. The RS symbol may be acomplex-valued symbol.

An information stream, an RS sequence, and an RS pattern are allocatedto each of the R spatial layers. An IS #r and an RS #r are allocated toa spatial layer #r (where r=1, . . . , R). Herein, r denotes a spatiallayer index indicating a spatial layer. An RS pattern allocated to thespatial layer #r is a time-frequency resource pattern used for RS #rtransmission.

The precoder 430 is configured to generate Nt transmit streams byperforming precoding on R spatial streams. The R spatial streams can beexpressed by SS #1, SS #2, . . . , SS #R. The Nt transmit streams can beexpressed by TS #1, TS #2, . . . , TS #Nt.

Each of the R spatial streams corresponds to one spatial layer. That is,an SS #r corresponds to a spatial layer #r (where r=1, . . . , R). Eachof the R spatial streams is generated based on an information stream, RSsequence, RS pattern allocated to a corresponding spatial layer. Thatis, the SS #r is generated based on an RS pattern allocated to the IS#r, the RS #r, and the spatial layer #r.

FIG. 17 is a block diagram showing an exemplary apparatus for wirelesscommunication using a precoded dedicated RS.

Referring to FIG. 17, a transmitter 500 includes a precoder 510 and Nttransmit antennas 590-1, . . . , 590-Nt. The precoder 510 is coupled tothe Nt transmit antennas 590-1, . . . , 590-Nt. A receiver 600 includesa channel estimation unit 610 and Nr receive antennas 690-1, . . . ,690-Nr. The transmitter 500 may be a part of a BS, and the receiver 600may be a part of a UE.

A MIMO channel matrix H is formed between the Nt transmit antennas590-1, . . . , 590-Nt and the Nr receive antennas 690-1, . . . , 690-Nr.The MIMO channel matrix H has a size of Nr(Nt. If the number of receiveantennas is 1, the MIMO channel matrix is a row vector. In general, amatrix conceptually includes a row vector as well as the column vector.

R spatial streams are input to the precoder 510. Each of the R spatialstreams includes a plurality of spatial symbols. The spatial symbol maybe a complex-valued symbol. A spatial symbol #k of an SS #r can beexpressed by x_(r)(k) (r=1, 2, . . . , R). The spatial symbol #k of theR spatial streams can be expressed by a spatial symbol vectorx(k)=[x₁(k) x₂(k) . . . x_(R)(k)]^(T). Herein, [•]^(T) denotes atransposed matrix of [•], and k denotes a time-frequency resource indexindicating a time-frequency resource for transmitting the spatial symbolvector. For example, the time-frequency resource indicted by k may be asubcarrier or a resource element.

x_(r)(k) is determined according to an RS pattern allocated to a spatiallayer #r. x_(r)(k) may be an information symbol of the SS #r or an RSsymbol of an RS #r according to the RS pattern. Alternatively, x_(r)(k)may be set to ‘ ’. As such, each of the R spatial streams is generatedon the basis of an information stream, an RS sequence, and an RS patternwhich are allocated to corresponding spatial layers.

The precoder 510 can perform precoding as expressed by the followingequation.

z(k)=W·x(k)   [Math.6]

Herein, z(k)=[z₁(k) z₂(k) . . . z_(Nt)(k)]^(T) denotes a transmit symbolvector, W denotes a precoding matrix having a size of Nt×R, andx(k)=[x₁(k) x₂(k) . . . x_(R)(k)]^(T) denotes a spatial symbol vector.Nt denotes the number of transmit antennas, and R denotes a rank. If therank is 1 (i.e., R=1), the precoding matrix is a column vector.

The transmitter 500 transmits a transmit symbol vector z(k) through theNt transmit antennas 590-1, . . . , 590-Nt.

In case of MU-MIMO, R spatial layers are allocated to K UEs (2(K(R,where K is a natural number). In case of MU-MIMO, the precoding matrixcan be regarded as a MU-MIMO precoding matrix. If W is the MU-MIMOprecoding matrix, a BS may create the matrix W by reconfiguring channelstate information (CSI) fed back from each of the K UEs. Alternatively,the BS may randomly configure the matrix W by using the CSI fed backfrom each of the K UEs. The CSI denotes general information on adownlink channel. There is no particular restriction on the CSI. The CSImay include at least one of a channel quality indicator (CQI), aprecoding matrix indicator (PMI), and a rank indicator (RI). The CQIindicates an MCS level suitable for a channel. The PMI indicates aprecoding matrix suitable for the channel. The RI indicates a rank ofthe channel. The PMI may be a simple matrix index in a codebook.Alternatively, the PMI may be channel quantization information, channelcovariance matrix, etc.

As such, when a precoded RS is used, an RS symbol of an RS sequence foreach spatial layer is also precoded and transmitted together with aninformation symbol of an information stream.

The receiver 600 receives a receive signal vector y=[y₁ y₂ . . .y_(Nr)]^(T) through the Nr receive antennas 690-1, . . . , 690-Nr. Thereceive signal vector y can be expressed by the following equation.

$\begin{matrix}\begin{matrix}{y = {{HWx} + n}} \\{= {{Px} + n}}\end{matrix} & \left\lbrack {{Math}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

Herein, n=[n_(i) n₂ . . . n_(Nr)]^(T) denotes a noise vector, and P=HWdenotes a precoded channel matrix.

The channel estimation unit 610 may estimate a precoded channel matrix Pfrom the received signal vector on the basis of the precoded dedicatedRS. When the precoded channel matrix P is estimated, the receiver 600may estimate an information stream transmitted for each spatial layer.Even if the receiver 600 cannot know the precoding matrix W and thuscannot estimate the MIMO channel matrix H, the receiver 600 candemodulate information by estimating the precoded channel matrix P.

When the precoded dedicated RS is used as described above, thetransmitter does not have to report the receiver a precoding matrix usedfor transmission. The receiver can explicitly demodulate informationeven if the receiver does not know the precoding matrix. When theprecoded dedicated RS is used, the transmitter does not have to limitthe precoding matrix. In general, the precoded dedicated RS is used toimplement non-codebook based precoding.

Precoding may be performed using one precoding matrix throughout a fullfrequency band. This is referred to as wideband precoding. In this case,one precoding matrix is used for one UE.

Meanwhile, a channel may be either a frequency selective channel or afrequency flat channel. Whether the channel is the frequency selectivechannel or the frequency flat channel may be determined on the basis ofa coherent bandwidth. The coherent bandwidth is inverse proportional toa delay spread.

In case of the frequency selective channel, a property of a MIMO channelmay vary depending on a frequency band. As long as a spatial channelcorrelation is relatively low, a different precoding matrix may be usedaccording to the frequency band to obtain a higher performance gain.

Frequency selective precoding is precoding performed using a differentprecoding matrix according to the frequency band. In this case, amultiple precoding matrix may be used for one UE. When the multipleprecoding matrix is used together with the precoded dedicated RS, thededicated RS has to be precoded using a precoding matrix correspondingto the frequency band. The frequency selective precoding may also applynot only to the frequency selective channel but also to the frequencyflat channel.

When demodulation is performed using the precoded dedicated RS, thereceiver performs channel estimation only in a resource block allocatedfor information reception. If the receiver is a part of the UE, the UEcan know the resource block allocated for information reception by usinga resource allocation field included in a DL grant. One or more resourceblocks may be allocated to the receiver. When a plurality of resourceblocks are allocated, the plurality of resource blocks may be allocatedeither consecutively or non-consecutively.

When the wideband precoding is used, the receiver can perform channelestimation by using channel interpolation throughout the allocatedresource block. In case of using the frequency selective precoding, theplurality of precoding matrices can be used in the resource blockallocated to the receiver. When the receiver cannot know a frequencyregion in which a coherent precoding matrix is used, the receiver canestimate a channel on a resource block basis. However, since the channelinterpolation cannot be performed throughout a plurality of resourceblocks, channel estimation performance may deteriorate. If the receivercan know the frequency region in which the coherent precoding matrix isused, the receiver can perform channel estimation by using the channelinterpolation in the frequency region in which the coherent precodingmatrix is used. If the channel is estimated by using the channelinterpolation, noise and interference can be suppressed, thereby capableof increasing channel estimation performance.

Therefore, the receiver needs to know information on the frequencyregion in which the same precoding matrix is used. The frequency regionin which the same precoding matrix is used may be pre-agreed between thetransmitter and the receiver. Alternatively, the transmitter may reportto the receiver the frequency region in which the same precoding matrixis used.

FIG. 18 is a flowchart showing a signal transmission method in awireless communication system according to an embodiment of the presentinvention.

Referring to FIG. 18, a BS indicates precoding bandwidth information toa UE (step S110). The precoding bandwidth information is information ona frequency region in which a coherent precoding matrix is used. Thefrequency region in which the coherent precoding matrix is used may alsobe referred to as a precoding subband. That is, precoding matrices areidentical in the precoding subband. For example, the precoding subbandmay be a plurality of consecutive resource blocks or a plurality ofconsecutive resource elements (or subcarriers). The precoding bandwidthinformation may indicate a size of the precoding subband. A precodinggranularity may be determined according to the size of the precodingsubband.

The BS may indicate the precoding bandwidth to the UE either explicitlyor implicitly. The BS may explicitly indicate the precoding bandwidthinformation by using physical layer signaling or higher layer signalingsuch as RRC signaling. In case of the physical layer signaling, theprecoding bandwidth information may be transmitted over a PDCCH. In thiscase, the precoding bandwidth information may be included in a DL grant.

The BS transmits a precoded signal to the UE (step S120). The precodedsignal is a signal obtained by precoding an RS for each spatial layerand information for each spatial layer.

The UE estimates a channel on the basis of the RS for each spatiallayer, and demodulates the information for each spatial layer (stepS130).

In a frequency division duplex (FDD) scheme, the BS cannot know adownlink channel property. The UE estimates a downlink channel, andfeeds back CSI for the downlink channel property over a feedbackchannel. In this case, the UE can estimate the downlink channel by usinga common RS such as a CSI-RS.

In a time division duplex (TDD) scheme, channel reciprocity exists inwhich an uplink channel property and a downlink channel property arealmost reciprocal. In case of using the TDD scheme, the UE can also feedback the CSI for the downlink channel property.

The BS may use the fed back CSI in downlink transmission. The CSIincludes a PMI, and the BS may transmit information to the UE on thebasis of the fed back PMI. Such an information transmission mechanism isreferred to as a closed-loop mechanism. The closed-loop mechanismtransmits information in a channel adaptive manner, thereby improvingsystem performance.

The BS may not use the fed back CSI in downlink transmission. Such aninformation transmission mechanism is referred to as an open-loopmechanism. In case of the open-loop mechanism, the UE may not feed backthe PMI.

The frequency selective precoding may be used both in the closed-loopmechanism and the open-loop mechanism. In case of the closed-loopmechanism, a multiple precoding matrix may be used to optimize precodingperformance according to a frequency band. In case of the open-loopmechanism, the multiple precoding matrix may be used either randomly orin a predefined manner. Accordingly, a frequency diversity can increasewithout having to feed back specific spatial channel information such asthe PMI. In both the close-loop mechanism and the open-loop mechanism,it is preferable that the UE knows the precoding bandwidth information.

Hereinafter, a method of indicating precoding bandwidth information by aBS to a UE will be described.

1. Reuse of Feedback Subband Definition in Closed-Loop Mechanism

In the FDD scheme, each of wideband precoding and frequency selectiveprecoding may be associated with PMI feedback. CSI may be channel stateinformation for a full frequency band or channel state information for afeedback subband which is a part of the full frequency band.

Definition on the feedback subband may be reused for a precodingsubband. A size of the precoding subband is equal to a size of thefeedback subband.

The feedback subband may be a plurality of consecutive resource blocksor a plurality of consecutive resource elements (or subcarriers). Ingeneral, the feedback subband may be an aggregation of resource blocks.For example, the size of the feedback subband may be 4 resource blocks,8 resource blocks, or the like. The size of the feedback subband mayvary depending on a downlink transmission bandwidth.

The size of the feedback subband may be set by the BS. The BS may setthe size of the feedback subband by using a higher layer such as an RRC.Precoding bandwidth information is implicitly indicated by the feedbacksubband size set by the BS. Alternatively, the feedback subband size maybe pre-defined between the BS and the UE. In this case, the feedbacksubband size may be pre-defined according to the downlink transmissionbandwidth.

Examples of a PMI feedback type may include a single PMI type and amultiple PMI type. In case of the single PMI type, the UE may feed backone PMI throughout a full frequency band. In case of the multiple PMItype, the UE may feed back PMIs for respective feedback subbands. ThePMI feedback type may be set by using a higher layer such as an RRC.

In a case where the PMI feedback type is the multiple PMI type, thefeedback subband size can be pre-defined according to the downlinktransmission bandwidth.

The following table shows the feedback subband size with respect to thedownlink transmission bandwidth N_DL.

TABLE 1 Downlink Transmission Bandwidth Feedback Subband Size (N_DL) (k)6-7 NA  8-10 4 11-26 4 27-63 6  64-110 8

FIG. 19 shows an example of a feedback subband when using the single PMItype.

Referring to FIG. 19, a downlink transmission bandwidth N_DL is 12. Afull frequency band includes 12 resource blocks (RBs), i.e., RB #1, RB#2, . . . , RB #12. It is assumed that the PMI feedback type is set tothe single PMI type by using higher layer signaling. A feedbackbandwidth is the full frequency band. The UE feeds back one PMIthroughout the full frequency band.

FIG. 20 shows an example of a precoding subband when using the singlePMI type.

Referring to FIG. 20, RB #4, RB #8, RB #9, and RB #11 are RBs scheduledfor the UE. The RB #4, RB #8, RB #9, and RB #11 are allocated to the UEto receive information. Information on the RBs allocated to the UE maybe included in a DL grant. As such, a plurality of RBs may be allocatedto the UE in a non-consecutive manner.

It is assumed that the PMI feedback type is set to the single PMI typeby using higher layer signaling. It is also assumed that definition on afeedback subband is reused for the precoding subband. In this case, aprecoding bandwidth is the full frequency band. Therefore, the UE canperform channel estimation by using channel interpolation throughout allallocated RBs, i.e., RB #4, RB #8, RB #9, and RB #11.

When multiple carriers are supported, it can be assumed that the sameprecoding matrix is used for the full frequency band in one carrier.

FIG. 21 shows an example of a feedback subband when using the multiplePMI type.

Referring to FIG. 21, a downlink transmission bandwidth N_DL is 12. Afull frequency band includes 12 RBs, i.e., RB #1, RB #2, . . . , RB #12.It is assumed that the PMI feedback type is set to the multiple PMI typeby using higher layer signaling. Referring to Table 1, a feedbacksubband size is 4. Therefore, a feedback bandwidth is 4 RBs. The UEfeeds back one PMI for every 4 RBs.

FIG. 22 shows an example of a precoding subband when using the multiplePMI type.

Referring to FIG. 22, RB #1, RB #2, and RB #11 are RBs scheduled for theUE. It is assumed that the PMI feedback type is set to the multiple PMItype by using higher layer signaling. It is also assumed that definitionon a feedback subband is reused for the precoding subband. Since afeedback subband size is 4, the precoding subband corresponds to 4 RBs.The UE may expect that a single precoding matrix is used in theprecoding subband. Therefore, the UE can perform channel estimation byusing channel interpolation in the precoding subband.

The RB #1 and the RB #2 are RBs included in one precoding subband, andthe RB #11 is an RB included in another precoding subband. Therefore,the UE can perform channel estimation by using channel interpolationthroughout the RB#1 and the RB#2. When channel estimation is performedon the RB #11, the UE does not perform channel interpolation on the RB#1 and the RB #2.

2. Separate Precoding Bandwidth

Even if the BS receives a PMI fed back from the UE, the BS may useanother precoding matrix according to a preference of the BS. In thiscase, a precoding subband may be defined independently from a feedbacksubband. A precoding bandwidth may be defined variously. Precodingbandwidth information reported by the BS to the UE may be a precodingsubband index indicating the precoding bandwidth.

The following table shows an example of the precoding bandwidthdepending on the precoding subband index when 2 bits are used to expressthe precoding subband index.

TABLE 2 Precoding Subband Index Precoding BW(# of RB) 0 4 1 6 2 8 3 12

The following table shows an example of the precoding bandwidthdepending on the precoding subband index when 3 bits are used to expressthe precoding subband index.

TABLE 3 Precoding Subband Index Precoding BW(# of RB) 0 2 1 4 2 6 3 8 410 5 12 6 14 7 16

A maximum precoding bandwidth may be a full frequency band. In thiscase, the precoding subband index may indicate wideband precoding. Thefollowing table shows an example of the precoding bandwidth depending onthe precoding subband index.

TABLE 4 Precoding Subband Index Precoding BW(# of RB) 0 2 1 4 2 6 3 8 410 5 12 6 14 7 Wideband precoding

A minimum precoding bandwidth may be one RB. The following tableindicates another example of the precoding bandwidth depending on theprecoding subband index.

TABLE 5 Precoding Subband Index Precoding BW(# of RB) 0 1 1 4 2 6 3 8 410 5 12 6 14 7 Wideband precoding

An N-bit precoding subband index may be transmitted over a PDCCH bybeing included in a DL grant. Alternatively, the N-bit precoding subbandindex may be transmitted by using higher layer signaling.

3. Precoding Bandwidth Information in Open-Loop Mechanism

In case of the open-loop mechanism, the UE does not have to feed back aPMI. In case of the open-loop mechanism, a higher diversity gain isrequired in comparison with the closed-loop mechanism. As one example ofa diversity mode, precoding matrix switching (PMS) may be used toincrease a diversity gain. The PMS may be implemented with matrices in acode book. The matrices may vary depending on a precoding subband. Theprecoding subband may be defined with one or more RB levels. Theprecoding matrix may vary even in one RB. In this case, a precodingbandwidth may be defined with one or more resource element levels. Forexample, the precoding bandwidth may be 6 resource elements.

The precoding bandwidth may be determined according to a transmissionscheme. For example, if open-loop spatial multiplexing is determined asthe transmission scheme, the precoding bandwidth may be determined withk RBs. k may be determined by the BS or may be pre-defined. If k isdetermined by the BS, the BS may indicate k to the UE by using physicallayer signaling or higher layer signaling. If the closed-loop spatialmultiplexing is determined as the transmission scheme, widebandprecoding may be used.

The following table shows an example of the precoding bandwidthaccording to the transmission scheme.

TABLE 6 Transmission Scheme Precoding BW(# of RB) 6-7  k 8-10 Widebandprecoding

4. Feedback Confirmation

A confirmation bit may be used for frequency selective precodingtogether with a dedicated RS. The confirmation bit may indicate whethera precoding subband is identical to a feedback subband. If the BSindicates PMI feedback for each feedback subband, the confirmation bitmay be used to determine whether to apply the frequency selectiveprecoding in which the precoding subband is identical to the feedbacksubband. The UE can know precoding bandwidth information by using theconfirmation bit. The BS may transmit the confirmation bit to the UE byusing physical layer signaling or higher layer signaling.

5. Unified Mode

In both the closed-loop mechanism and the open-loop mechanism, the BScan indicate a precoding bandwidth by using a precoding subband index.Tables 2 to 5 may be used by reference for the precoding bandwidthdepending on the precoding subband index. However, this is for exemplarypurposes only, and thus the precoding bandwidth depending on theprecoding subband index is not limited thereto. The precoding subbandindex may be transmitted to the UE by using physical layer signaling orhigher layer signaling.

In case of the open-loop mechanism, the BS may limit an availableprecoding bandwidth. The precoding bandwidth available in the open-loopmechanism may be a subset of precoding bandwidths available in theclosed-loop mechanism. As shown in the following table, the availableprecoding bandwidth may be limited when using the open-loop mechanism.

TABLE 7 Precoding Subband Index Precoding BW(# of RB) 0  1(open-loop,closed-loop) 1  4(open-loop, closed-loop) 2  6(closed-loop only) 3 8(closed-loop only) 4 10(closed-loop only) 5 12(closed-loop only) 614(closed-loop only) 7 Wideband precoding(open-loop, closed-loop)

6. Distributed Resource Allocation

The BS may allocate a DL time-frequency resource to the UE in adistributed manner. The time-frequency resource may be an RB. Among RBsallocated to the UE, consecutive RBs are referred to as an RB group.Multiple RB groups may be allocated to the UE. The RB groups are dividedin a frequency domain.

A precoding bandwidth may start from a first RB of the RB group. Theprecoding bandwidth may be identical to the RB group. In this case, theUE may obtain precoding bandwidth information by using a resourceallocation field included in a DL grant. Accordingly, the BS mayflexibly allocate an RB to the UE. In addition, an interpolation gaincan be maximized.

FIG. 23 shows an example of a precoding bandwidth.

Referring to FIG. 23, RB #4, RB #5, RB #9, RB #10, and RB #11 are RBsscheduled for the UE. The RB #4 and the RB #5 belong to an RB group #1.The RB #9, the RB #10, and the RB #11 belong to an RB group #2. Aprecoding bandwidth #1 is identical to the RB group #1. Therefore, theUE can perform channel interpolation in the RB group #1. A precodingbandwidth #2 is identical to the RB group #2. Therefore, the UE canperform channel interpolation in the RB group #2.

7. Support of Rank-Specific Precoding Bandwidth

A precoding bandwidth may be supported only in a rank specific manner.For example, if a system supports up to rank-8 transmission, theprecoding bandwidth may be reported only for a rank 5 or below.Accordingly, flexible scheduling is possible for a higher rank greaterthan or equal to a specific rank. In addition, a channel estimation gaincan be provided in a lower rank less than or equal to the specific rank.

8. Support of Layer-Specific Precoding Bandwidth

If a multiple spatial stream is transmitted by using a multiple spatiallayer, precoding bandwidth indication may be effective only in aspecific spatial layer. One RB-based precoding bandwidth may be used inanother spatial layer.

FIG. 24 is a block diagram showing an apparatus for wirelesscommunication for implementing an embodiment of the present invention. ABS 50 includes a processor 51 and an antenna 59.

The processor 51 is coupled to the antenna 59, and implements theproposed functions, processes, and/or methods. Layers of a protocolstack may be implemented by the processor 51. The antenna 59 transmitsor receives a signal. One or a plurality of antennas 59 may be provided.The BS 50 may further include a memory (not shown). The memory (notshown) is coupled to the processor 51, and stores a variety ofinformation for driving the processor 51.

A UE 60 includes a processor 61 and an antenna 69. The processor 61 iscoupled to the antenna 69, and implements the proposed functions,processes, and/or methods. Layers of a radio interface protocol may beimplemented by the processor 61. The antenna 69 transmits or receives asignal. One or a plurality of antennas 69 may be provided. The UE 60 mayfurther include a memory (not shown). The memory (not shown) is coupledto the processor 61, and stores a variety of information for driving theprocessor 61.

The processors 51 and 61 may include an application-specific integratedcircuit (ASIC), a separate chipset, a logic circuit, a data processingunit, and/or a radio frequency (RF) unit for mutually converting abaseband signal and a radio signal. The proposed transmitter may beimplemented in the processors 51 and 61. The memory (not shown) mayinclude a read-only memory (ROM), a random access memory (RAM), a flashmemory, a memory card, a storage medium, and/or other equivalent storagedevices. When the embodiment of the present invention is implemented insoftware, the aforementioned methods can be implemented with a module(i.e., process, function, etc.) for performing the aforementionedfunctions. The module may be stored in the memory (not shown) and may beperformed by the processors 51 and 61. The memory (not shown) may belocated inside or outside the processors 51 and 61, and may be coupledto the processors 51 and 61 by using various well-known means.

Accordingly, an apparatus and method for effectively transmitting asignal in a wireless communication system are provided. A UE can obtainprecoding bandwidth information. The UE can perform channel estimationby using channel interpolation in a frequency region included in aprecoding bandwidth on the basis of the precoding bandwidth information.In this manner, the UE can obtain better channel estimation performance.Therefore, overall system performance can be improved.

Additional advantages, objectives, and features of the present inventionwill become more apparent to those ordinary skilled in the art uponimplementation of the present invention based on the aforementioneddescriptions or explanation. Moreover, other unexpected advantages maybe found as those ordinary skilled in the art implement the presentinvention based on the aforementioned explanations.

Although the aforementioned exemplary system has been described on thebasis of a flowchart in which steps or blocks are listed in sequence,the steps of the present invention are not limited to a certain order.Therefore, a certain step may be performed in a different step or in adifferent order or concurrently with respect to that described above.Further, it will be understood by those ordinary skilled in the art thatthe steps of the flowcharts are not exclusive. Thus, another step may beincluded therein or one or more steps may be deleted within the scope ofthe present invention.

Various modifications may be made in the aforementioned embodiments.Although all possible combinations of the various modifications of theembodiments cannot be described, those ordinary skilled in that art willunderstand possibility of other combinations. For example, thoseordinary skilled in the art will be able to implement the invention bycombining respective structures described in the aforementionedembodiments. Therefore, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A signal transmission method performed by a base station in awireless communication system, the method comprising:generating Rspatial streams each of which is generated on the basis of aninformation stream and a reference signal;generating N transmit streamson the basis of the R spatial streams and a precoding matrix (whereR<N);mapping the N transmit streams to at least one resource block; andgenerating N signals from the N transmit streams mapped to the at leastone resource block, and transmitting the N signals to a user equipmentthrough respective antennas,wherein the precoding matrix is determinedon the basis of the at least one resource block and a precodingbandwidth, and wherein the precoding bandwidth is a bandwidth in whichthe same precoding matrix is used.
 2. The method of claim 1, furthercomprising transmitting precoding bandwidth information indicating theprecoding bandwidth to the user equipment.
 3. The method of claim 1,further comprising transmitting a downlink grant comprising a resourceblock allocation field indicating the at least one resource block to theuser equipment.
 4. The method of claim 3, wherein the downlink grantfurther comprises precoding bandwidth information indicating theprecoding bandwidth.
 5. The method of claim 2, wherein the precodingbandwidth information is transmitted by using higher layer signaling. 6.The method of claim 1, further comprising configuring a feedback type bywhich a feedback subband is determined for the user equipment, whereinthe precoding bandwidth is the same as the feedback subband.
 7. Themethod of claim 1, wherein the at least one resource block comprises afirst resource block group and a second resource block group, each ofthe first resource block group and the second resource block groupcomprises at least one contiguous resource block in a frequency domain,and the second resource block group is non-contiguous to the firstresource block group in the frequency domain,wherein a first precodingmatrix is used in the first resource block group, and wherein a secondprecoding matrix is used in the second resource block group.
 8. Achannel estimation method performed by a user equipment in a wirelesscommunication system, the method comprising:obtaining precodingbandwidth information indicating a bandwidth in which the same precodingmatrix is used;receiving a precoded signal by using at least oneresource block from a base station; and estimating a channel on thebasis of the precoding bandwidth information.
 9. A signal transmissionapparatus comprising:N antennas; and a processor coupled to the Nantennas and configured for transmitting precoding bandwidth informationindicating a bandwidth in which the same precoding matrix isused;generating R spatial streams each of which is generated on thebasis of an information stream and a reference signal;generating Ntransmit streams on the basis of the R spatial streams and a precodingmatrix;mapping the N transmit streams to at least one resource block;and generating N signals from the N transmit streams mapped to the atleast one resource block, and transmitting the N signals to a userequipment through respective antennas,wherein the precoding matrix isdetermined on the basis of the at least one resource block and theprecoding bandwidth information.